Initial Adsorption of C 60 Molecules on Si(111)-7 × 7 Surface with Al Nanocluster Array

The initial adsorption of C 60 molecules on an Al nanocluster array template, an Si(111)-7 £ 7 surface with an Al nanocluster array, was observed at various temperatures using a variable temperature scanning tunneling microscope (STM). STM images showed that C 60 molecules stay above the dimer lines near the corner hole, with one of the 6–6 bonds facing toward the adsorbing surface and forming chemical bonds with two neighboring corner Si adatoms. The strong interaction immobilized C 60 molecules on the adsorbing template. [DOI: 10.1380/ejssnt.2010.354]


I. INTRODUCTION
Since their discovery in 1985, C 60 molecules have attracted much attention in many scientific disciplines because of their interesting properties, such as spherical shape, high symmetry, and potential application for electrical and optical devices [1][2][3][4][5]. In view of its technical importance in industry, the adsorption of C 60 molecules on Si surfaces has been extensively studied [6][7][8][9][10][11][12][13][14][15][16][17]. C 60 molecules are typically chemisorbed on a reconstructed silicon surface through covalent bonds with the dangling bonds of the silicon substrate [16]. The distribution of C 60 molecules on the adsorbing surface strongly depends on the dangling bonds on the surface. Studies [7,8] have shown that C 60 molecules randomly adsorb on a pristine Si(111)-7×7 surface, whose centers are on rest atom sites, corner hole sites or dimer line sites. Passivation of the dangling bonds with SiO 2 [9], H 2 [12], and B [17] atoms increased the mobility of C 60 molecules and led to a more orderly distribution on the Si surface. The Si(111) surface was fully passivated and the energy difference on the surface almost vanished. It is therefore worth studying the deposition of C 60 molecules on a highly-ordered periodically passivated Si(111) surface.
An Al nanocluster array on Si(111)-7×7 surface is considered to be a potential template for constructing highlyordered artificial nano structures [18]. Large-scale perfect Al nanocluster arrays of identical cluster size and spacing even up to step edges [18][19][20][21][22][23][24][25] can be formed, providing an ideal periodic template for constructing functional nanostructures. Through STM images and first-principles total * This paper was presented at SSSJ-A3 Foresight Joint Symposium on Nanomaterials and Nanostructures, Hongo Campus, The University of Tokyo, Tokyo, Japan, 7-9 July, 2010. † Corresponding author: miki.kazushi@nims.go.jp energy calculation, Jia et al. identified the atomic structure of Al nanoclusters, the Al 6 Si 3 atomic model [19,20]. In our previous studies, the thermal stability of the Al nanocluster array itself, interaction between Al nanoclusters, and the formation process of Al nanoclusters and Al nanocluster arrays were investigated with a variable temperature STM up to around 600 • C. It was found that the Al nanocluster array is stable up to 500 • C, and transforms into √ 3× √ 3-Al phase above 500 • C [23]. From the activation energy for the diffusion of Al nanoclusters, it was concluded that an Al atom diffusion process, rather than an Al cluster diffusion process, creates the Al nanocluster array [24]. An attractive interaction between Al nanoclusters was found, which is responsible for the honeycomb structure [25]. By a combination of STM observation and Density Function Theory (DFT) calculations, Al trimers were found to be the key precursors in the formation process of the Al nanocluster. Therefore, due to the identical size of each nanocluster, periodic symmetry, single atom thickness, and also its thermal stability, a perfect 2D Al nanocluster array could be a potential template or artificial interface for growing subsequent nanostructures on Si substrate.
In the present work, we examined the initial adsorption of C 60 molecules on the Al nanocluster array template with a variable temperature STM, particularly the adsorption sites on the template and the interaction between C 60 molecules and adsorbing substrate, through STM images at various temperatures. The diffusion of C 60 molecules on the template is also discussed.

II. EXPERIMENTAL
The experiments were performed with a JEOL 4500 variable temperature STM combined with E-gun evaporator in an ultrahigh vacuum chamber (base pressure 1×10 −8 Pa). Samples were heated in the STM chamber e-Journal of Surface Science and Nanotechnology by direct current through the sample itself, and the substrate temperature was monitored by an infrared pyrometer. A chemically etched tungsten tip was used to observe STM images. Si(111) samples were cleaned briefly by HF acid solution and degassed at about 600 • C for 12 hours in the chamber. A clean Si(111)-7×7 surface with few defects was obtained by flashing to about 1150 • C several times and decreasing the temperature very slowly. An E-gun evaporator of Al with purity of 99.99% was used to grow an Al nanocluster array in the chamber, and a Knudsen-Cell was used to deposit C 60 molecules. A nearly perfect Al nanocluster array was prepared at 450 • C. After preparing the Al nanocluster array, C 60 molecules were deposited at substrate temperatures of room temperature, 200 • C, 300 • C, 380 • C and 500 • C.

III. RESULTS AND DISCUSSIONS
C 60 molecules were deposited onto the Al nanocluster array at 380 • C, the same temperature used for evaporating C 60 molecules from the Knudsen-Cell. Figure 1 shows STM images at various scales taken at 380 • C. The large bright protrusions are C 60 molecules adsorbing on the Si(111)-7×7 surface. Figure 1(a) shows that the distribution of C 60 molecules on the surface is isolated on the surface.
Step edges are not favored as adsorption sites and no large island nucleation is observed, neither on the terraces nor at the steps and kinks. We think that C 60 molecules simply adsorbed at the sites where they randomly landed on the surface. This adsorption would mean that there is a strong interaction between C 60 molecules and the adsorbing substrate. The magnified STM image in Fig. 1(b) shows that some dimers and trimers formed during the deposition, which stayed at the sites near the same corner hole of the Si(111)-7×7 unit cell. The clear background in the image indicates that no reconstruction occurred after the deposition; the surface retained its own structure after the deposition of C 60 molecules. The further magnified STM image in Fig. 1(c) shows four bright stripes, which are thought to be related to the internal structure of a single adsorbing C 60 molecule [7,15]. Usually, C 60 molecules continue to self-rotate on the inert surface above room temperature. Under STM, they look like bright balls and the internal structure can not be observed; the internal structure can be observed only when the self-rotation is halted by the strong interaction between the C 60 molecule and the substrate or by low temperature [15]. The fact that we could observe the internal structure during our experiment confirms the strong interaction between the C 60 molecules and the adsorbing substrate.
The adsorption sites of C 60 molecules were studied by carefully analyzing the STM images. To understand clearly the adsorption sites of C 60 molecules, half unit cells of the Si(111)-7×7 surface are drawn in Fig. 2(a). It can be seen that C 60 molecules adsorbed at the site between two neighboring Al nanoclusters, which is near the corner hole of the half unit cell, not above the Al adatoms in the Al nanocluster. Because of the large size of the C 60 molecule, it is not obvious where the bonding sites are. Two unit cells of the schematic structure of the Al nanocluster array are overlaid on the STM image in Fig. 2(a) to determine the adsorption sites of the C 60 molecules. Careful inspection of the series of STM images shows that C 60 molecules stay above the dimer lines near the corner hole and have bonds with two neighboring Si corner adatoms in different half unit cells, as indicated in the schematic picture on the right in Fig. 2(b). It is known that for the Si(111)-7×7 surface, there are 19 dangling bonds in the unit cell. After the deposition, C 60 molecules form chemical bonds with these dangling bonds. This interaction was characterized by the charge transfer from the dangling bond surface states into the lowest unoccupied molecular orbital (LUMO) of C 60 molecules [12,13]. C 60 molecules adsorb on the rest atom site, corner hole site and dimer line site on the Si(111)-7×7 surface, having bonds with the neighboring Si adatoms [7,8]. After the formation of the Al nanocluster array, the dangling bonds of center adatoms and rest atoms are passivated by Al atoms, but there are still 7 dangling bonds: 6 dangling bonds from the corner adatoms and 1 dangling bond from the atom in the corner hole ( Fig. 2(b)). Once C 60 molecules have adsorbed on the Al nanocluster array template, they will interact with these remaining dangling bonds. We think that there is similar interaction between C 60 molecules and the dangling bonds from the Si corner adatoms, namely, the charge transfer from the dangling bond surface states into the LUMO of C 60 molecules. Since C 60 molecules are large, they will interact with two neighboring corner adatoms in different half unit cells, as indicated by the dashed ring in Fig. 2(b). Because of the distance and spatial hindrance, the dangling bond in the corner hole is not included in this kind of interaction.
The internal structure of the single adsorption of C 60 molecules in Fig. 1(c) shows the two-fold symmetry, which is the same as reported STM images [7]. In our experiment, the half part near the Al nanocluster appears brighter than the half part near the corner hole because the electric density near the Al nanocluster is higher. As Kroto et al. reported [1], C 60 is a molecule with high symmetry, having twelve pentagons and twenty hexagons. There are two kinds of bonds in C 60 molecules: 5-6 bonds between a pentagon and a hexagon, and 6-6 bonds between two hexagons. Considering the model of C 60 molecules, if C 60 molecules adsorb on the surface with one of the pentagons or hexagons facing towards the dimer lines and two dangling bonds, the adsorption structure will be three-fold symmetry or five-fold symmetry. If C 60 molecules adsorb on the surface with one of the 5-6 bonds facing toward the adsorbing surface, there is no symmetric axis for this kind of adsorption structure. The adsorption structure will be two-fold symmetry only when C 60 molecules adsorb on the surface with one of the 6-6 bonds facing toward the adsorbing surface, which agrees with our STM images. Based on the above discussion, we suggest that C 60 molecules adsorb on the surface with one of the 6-6 bonds facing toward the adsorbing surface, forming bonds with the dangling bonds from the two neighboring corner Si adatoms. Passivation of the Al nanocluster array changes the adsorption of C 60 molecules on the Si(111)-7×7 surface, and there is only one kind of adsorption site on the surface of the Al nanocluster array template.
The diffusion of C 60 molecules was further studied by STM for samples prepared at various temperatures, such as room temperature, 200 • C and 300 • C, even up to 500 • C, at which Al nanoclusters are not stable, and consecutive images were taken at the temperatures after the preparation. However, we found that C 60 molecules remained in their own sites and there was no aggregation during the annealing. Figures 3(a) and 3(b) are examples of consecutive images at 380 • C. These two images were taken from one STM movie in the same area, as shown by the green square. These two images clearly demonstrate that C 60 molecules remain in the same position even after 22 minutes. The immobility of the C 60 molecules also proved that the interaction between the C 60 molecules and the Al nanocluster array template is by strong chemical bonding.

IV. CONCLUSIONS
We studied the initial adsorption of C 60 molecules at various temperatures by STM. C 60 molecules tend to stay above the dimer lines, rather than in the corner hole or above the adatoms in the Al clusters. STM images of individual C 60 molecules suggest that C 60 molecules adsorb on the surface with one of the 6-6 edges facing toward the surface and forming chemical bonds with the dangling bonds from two neighboring Si corner adatoms. Consecutive STM images at various temperatures showed that C 60 molecules are immobile on the Al nanocluster array template. The immobility of C 60 molecules further proves the strong interaction between C 60 molecules and the Al nanocluster array template. Due to the immobility, C 60 molecules can not form monolayer or well-ordered clusters on the Al nanocluster array template as they do on many metal surfaces.
for the Promotion of Science. This work was also supported in part by World Premier International Research Center (WPI) Initiative on Materials Nanoarchitectonics (MANA), MEXT, Japan.